1. Field of the Invention
This invention relates to a method for the production of an unsaturated carboxylic ester from a corresponding unsaturated carboxylic acid and an aliphatic alcohol and more particularly to a method for the production of an unsaturated carboxylic ester which is enabled, by controlling the temperature of a heat source supplied from a heating means and the temperature of a reaction solution in respectively prescribed ranges, to prevent a polymerization on account of a raw material compound.
2. Description of the Related Art
In the production of an ester from an unsaturated carboxylic acid and an alcohol, the reaction vessel for use in the production is equipped with a heating means generally because the relevant reaction is enhanced by heating the reaction solution. In the method for producing an ester from an unsaturated carboxylic acid and an alcohol, the esterification is prevented from proceeding fully satisfactorily and the conversion ratio is lowered because of by-produced water. For this reason, the reaction solution requires to purge itself of the by-produced water contained therein. A method which consists in expelling the by-produced water from the reaction solution by means of a distillation column in order to shift the reaction equilibrium toward the reaction side and increase the conversion ratio is now available. In execution of this method, the reaction solution must be heated so as to retain in a boiling state and the reaction vessel must be furnished with a heating means.
Generally, the fact that the heating temperature is high is at an advantage in reducing the cost of equipment and the cost of labor because of decreasing the heating surface area and curtailing the temperature increasing time. Thus, a method which consists in relying on a heating means such as a heating jacket which is capable of covering the outer periphery of the reaction vessel to supply a heating medium having a higher temperature than the target temperature of the reaction solution thereby heating the reaction solution in the reaction vessel to a prescribed temperature has been finding in extensive utility.
Some of the reactions of esterification are catalyzed by the use of an ion-exchange resin. Particularly when the ion-exchange resin is uniformly dispersed in the reaction solution for the purpose of promoting the reaction of esterification, the ion-exchange resin is kept stirred by means of an agitating element equipped in the reaction vessel with the aim so as to proceed the reaction of esterification advantageously. To recover the product of the reaction which has been performed as described above, it becomes necessary to separate the ion-exchange resin from the reaction solution. Thus, a resin separating tank is adjoined to the reaction vessel and operated so as to introduce the reaction solution containing the product of the esterification and the ion-exchange resin, and keep the solution containing the resin standing at rest. The reaction vessel is operated meanwhile to permit circulation of the slurry containing the ion-exchange resin to the reaction vessel for the purpose of ensuring continuation of the manufacture of the product of the esterification. As a measure to attain this continued manufacture, the official gazette of JP-A-10-231,275, for example, discloses a method which reacts a carboxylic acid with an alcohol in a suspension of a strong acidic cation-exchange resin as a catalyst, introduces slurry reaction solution containing the ion-exchange resin into a resin separating tank, separates the slurry reaction solution into a clear liquid part and a slurry thick liquid part in the resin separating tank, and subsequently withdraws the clear liquid through the upper part of the resin separating tank and meanwhile withdraws the slurry thick liquid through the lower part of the resin separating tank in order to circulate the resin to the reaction vessel. When the ascending speed of the clear liquid based on the cross section of the resin separating tank is set below a specific level, the clear liquid which is obtained by this method either contains substantially no ion-exchange resin or contains the ion-exchange resin only in a markedly lowered concentration. Besides, it is claimed that this method, by an additional measure of causing a pipe extending from the reaction vessel to the resin separating tank to be connected to the lower half part of the separating tank at an angle in the range of 20-70° C. relative to the horizontal line, is enabled to avoid disturbing the flow of the liquid near the inlet of the separating tank and inducing flotation of the ion-exchange resin and allow easy separation of the slurry reaction solution into the slurry thick liquid and the clear liquid.
When the unsaturated carboxylic acid happens to be a compound such as an acrylic acid which has a polymerizable unsaturated double bond, however, there may arise the possibility that the heating means equipped on the outer periphery of the reaction vessel will excessively heat the outer wall of the reaction vessel and will consequently induce formation of a polymer on the inner wall of the reaction vessel. Particularly when an ion-exchange resin is used as a catalyst for the reaction, there may arise the possibility that the stirring of the reaction solution for the purpose of uniformly mixing the resin with the reaction solution will cause the splashing on the inner wall of the reaction vessel with the reaction solution, and polymerization. This adhesion of the polymer may possibly cause a loss in the raw material compound and the target product. When the ester is continuously produced, therefore, the adhered polymer results in inducing a large decline of the yield of the product. The polymer further impairs the flow of the liquid in the pipe, blocks the pipe, and entails the necessity of periodically suspending the operation of the apparatus for production and purging the reaction vessel and the pipes of the polymer adhering to their interiors.
When the resin separating tank is additionally installed as a means for the separation of the ion-exchange resin, it is preferred to have a pipe interconnect the bottom of the resin separating tank and the reaction vessel, make the separated resin circulate to the reaction vessel, and keep the amount of the resin in the reaction vessel constant. The use of this pipe, however, possibly entails such problems as induce stagnation of resin within the pipe, decrease the amount of the resin circulated thereby to the reaction vessel, cause the pipe to block, and even suffer the resin to flow out into the next step of the process.
This invention, therefore, aims to provide a method for producing an ester to solve the technical problems incurred during the separation for removal of the by-produced water. More specifically, this invention aims to provide a method for the production of the unsaturated aliphatic ester, which method is capable of efficiently producing the ester of high quality as preventing the polymerization which may well be called a prime cause of the technical problems.